Java BigDecimal 区别

Question

我想看看是否有人可以解释为什么以下代码适用于 valueOf 但不适用于其他代码。

import java.math.BigDecimal; 
public class Change {
   public static void main(String args[]) {
     double a = 4.00d;
     double b = 3.10d;
     BigDecimal a1 = new BigDecimal(a);
     BigDecimal b1 = new BigDecimal(b);
     BigDecimal diff = a1.subtract(b1);
     System.out.println("Double difference");
     System.out.println(diff);

     float c = 4.00f;
     float d = 3.10f;
     BigDecimal a2 = new BigDecimal(c);
     BigDecimal b2 = new BigDecimal(d);
     BigDecimal diff2 = a2.subtract(b2);
     System.out.println("Float difference");
     System.out.println(diff2);

     System.out.println("Valueof Difference");
     System.out.println(BigDecimal.valueOf(4.00).subtract(BigDecimal.valueOf(3.10)));

   }
 }

输出如下:

>java Change
 Double difference
  0.899999999999999911182158029987476766109466552734375
 Float difference
  0.900000095367431640625
 Valueof Difference
 0.9

我的问题是:valueOf() 做了什么来获得精度？有没有其他方法可以在不手动四舍五入到 2 位数的情况下获得正确的结果？

谢谢，

Answer 1

查看 BigDecimal 的源代码，它确实:

public static BigDecimal valueOf(double val) {
    // Reminder: a zero double returns '0.0', so we cannot fastpath
    // to use the constant ZERO.  This might be important enough to
    // justify a factory approach, a cache, or a few private
    // constants, later.
    return new BigDecimal(Double.toString(val));
}

来自其 JavaDoc:

Translates a double into a BigDecimal, using the double's canonical string representation provided by the Double.toString(double) method.

Note: This is generally the preferred way to convert a double (or float) into a BigDecimal, as the value returned is equal to that resulting from constructing a BigDecimal from the result of using Double.toString(double).

由于浮点表示法，double 值与您设置的不完全相同。但是，在 String 表示期间，它会四舍五入它显示的内容。 (所有规则都在 JavaDoc 上)。

此外，由于这种舍入，如果您这样做:

BigDecimal d = BigDecimal.valueOf(4.00000000000000000000000000000000001));

你会得到错误的值。 (d == 4.0)

所以，用字符串初始化它们总是更好。